Gas-Sensing Performance of Metal Oxide Heterojunction Materials for SF6 Decomposition Gases: A DFT Study

The online monitoring of GIS equipment can be realized through detecting SF6 decomposition gasses. Metal oxide heterojunctions are widely used as gas-sensing materials. In this study, the structural and electrical properties of In2O3-ZnO and TiO2-ZnO heterojunctions were analyzed based on density functional theory calculations. After heterojunction structural optimization, the electrical conductivity of these two heterojunctions was enhanced compared to each intrinsic model, and the electrical conductivity is ranked as follows: In2O3-ZnO heterojunction > TiO2-ZnO heterojunction. The gas-sensing response of these two heterojunctions to four SF6 decomposition gasses, H2S, SO2, SOF2, and SO2F2, was investigated. For gas adsorption systems, the adsorption energy, charge transfer, density of states, charge difference density, and frontier molecular orbitals were calculated to analyze the adsorption and gas-sensing performance. For gas adsorption on the In2O3-ZnO heterojunction surface, the induced conductivity changes are in the following order: H2S > SO2F2 > SOF2 > SO2. For gas adsorption on the TiO2-ZnO heterojunction surface, H2S and SOF2 increase conductivity, and SO2 and SO2F2 decrease conductivity.


Introduction
In modern power systems, the gas insulated switchgear (GIS) is an essential critical piece of equipment to ensure the safe and stable operation of the power grid [1].Sulfur hexafluoride gas (SF 6 ) is widely used in GIS equipment due to its excellent insulation and arc extinguishing performance [2].However, during the long-term operation of GIS equipment, external environmental factors (such as overvoltage and high temperature) and inherent insulation defects of the equipment itself may lead to internal discharge and overheating faults [3]; furthermore, this can lead to SF 6 decomposition, which has a serious impact on GIS reliability.Therefore, in order to ensure the stable operation of GIS equipment, effective online monitoring measures must be taken to quickly identify and eliminate the potential insulation faults [4].Research has confirmed that SF 6 decomposition gasses mainly include H 2 S, SO 2 , SOF 2 , and SO 2 F 2 , and the operating state of the GIS device can be reflected by detecting the presence and concentration of these gasses [5,6].Therefore, it is necessary to explore gas-sensitive materials with high sensitivity and selectivity for SF 6 decomposition gasses detection.
In recent years, the adsorption research on SF 6 decomposition gas mainly focuses on graphene, two-dimensional metal sulfur compounds (TMDCs), metal oxides, and surfacemodified metal oxides [7][8][9].Among them, metal oxides are widely used materials with the characteristics of low cost, low power consumption, and easy synthesis for gas detection [10], including ZnO, TiO 2 , In 2 O 3 , SnO 2 , and WO 3 .ZnO is an n-type semiconductor, which is widely used for detecting toxic and hazardous gasses due to its wide bandgap and high electron mobility [11].At the same time, researchers have also proposed various forms of nanostructures, such as nanosheets [12], nanorings [13], nanowires [14], and nanorods [15], to improve the sensitivity, selectivity, and stability of the ZnO-based gas sensor, and to reduce the working temperature and reaction recovery time.Razavi et al. prepared ZnO@TiO 2 nanocomposites and verified their potential in the determination of hydrazine (N 2 H 4 ) [16].The research showed that TiO 2 exhibits high selectivity and sensitivity towards small gas molecules [17][18][19].Shooshtariet al. prepared an electronic nose based on carbon nanotube-titanium dioxide hybrid nanostructures for the detection and discrimination of volatile organic compounds (VOCs) [20].In addition, In 2 O 3 is also widely used in gas detection and monitoring due to its suitable bandgap and good chemical stability, and excellent gas-sensing performance [21,22].The gas-sensing property of In 2 O 3 can also be improved through surface metal modification or the formation of heterojunctions [23].However, intrinsic metal oxides have limited gas sensitivity to specific gasses, and metal oxide heterojunctions are often used to improve the gas sensitivity of metal oxides-based gas sensors due to Fermi level effects and synergistic effects between different metal oxides [24,25].Cheng et al. reported an ammonia sensor based on the ZnO/CuO heterojunction using a hydrothermal method, which effectively improved the gas-sensing property of the ZnO sensor for NH 3 at room temperature [26].
In order to improve the sensitivity and selectivity of gas-sensitive materials towards the SF 6 decomposition gasses of H 2 S, SO 2 , SOF 2 , and SO 2 F 2 , this study adopts In 2 O 3 -ZnO and TiO 2 -ZnO heterojunctions as gas-sensitive materials, and studies their gas-sensing properties to SF 6 decomposition gasses based on density functional theory (DFT).Firstly, the structural models of In 2 O 3 -ZnO and TiO 2 -ZnO heterojunctions were constructed and optimized, and then the structural and energy band properties were analyzed.Secondly, the adsorption structures of different SF 6 decomposition gasses on the heterojunction surface were constructed and optimized.Finally, by analyzing the density of states (DOS) and charge difference density (CDD) of gas adsorption systems, the adsorption performance of the metal oxide heterojunction materials for SF 6 decomposition gasses was analyzed.The research results play an important role in promoting the application of gas-sensing technology in the online monitoring of power insulation equipment.In heterostructures, there is a formation of chemical bonds between the In 2 O 3 layer and the ZnO layer, which implies a strong interaction force between the two metal oxides.In addition, its enormous formation energy (−18.017eV) and significant geometric deformation also prove that the formation between the two materials is highly stable.The energy band structure of the In 2 O 3 -ZnO heterojunction shown in Figure 1c intuitively displays the electron property of the heterojunction.According to the calculation results, the energy required for electrons to transition from the valence band to the conduction band is 0.652 eV.Compared to the intrinsic In 2 O 3 (1.538eV) and intrinsic ZnO heterojunction (1.769 eV), the significant decrease in energy indicates that the conductivity of the heterojunction is superior.

Adsorption Properties of In2O3-ZnO Heterojunction 
Gas adsorption structure analysis Figure 2 shows the most stable adsorption structures for H2S, SO2, SOF2, and SO2F2 on the In2O3-ZnO heterojunction surface, and Table 1 lists the corresponding adsorption parameters.For the H2S adsorption system shown in Figure 2 2(d1,d2), there is a significant distance between the gas molecules and heterojunctions; gas molecules can easily desorb from the surface of the In2O3-ZnO heterojunction, indicating that the adsorption process is physical adsorption.In addition, the small adsorption energy (SOF2: Eads = −0.465eV, SO2F2: Eads = −0.501eV) and the small amount of charge transfer (SOF2: Qt = 0.016 |e|, SO2F2: Qt = 0.011 |e|) also confirm this physical adsorption effect.In summary, the adsorption process of H2S and SO2 is chemical adsorption, and the molecular structure of the H2S gas molecules is destroyed, making it difficult to carry out the desorption process.The adsorption process of SOF2 and SO2F2 is physical adsorption and the adsorption energy is moderate, which together can have a certain adsorption capacity for gas molecules and make desorption possible.2(c1,c2,d1,d2), there is a significant distance between the gas molecules and heterojunctions; gas molecules can easily desorb from the surface of the In 2 O 3 -ZnO heterojunction, indicating that the adsorption process is physical adsorption.In addition, the small adsorption energy (SOF 2 : E ads = −0.465eV, SO 2 F 2 : E ads = −0.501eV) and the small amount of charge transfer (SOF 2 : Q t = 0.016 |e|, SO 2 F 2 : Q t = 0.011 |e|) also confirm this physical adsorption effect.In summary, the adsorption process of H 2 S and SO 2 is chemical adsorption, and the molecular structure of the H 2 S gas molecules is destroyed, making it difficult to carry out the desorption process.The adsorption process of SOF 2 and SO 2 F 2 is physical adsorption and the adsorption energy is moderate, which together can have a certain adsorption capacity for gas molecules and make desorption possible.

 DOS, CDD, and molecular orbital analysis
Figure 3 shows the density of states distribution of H2S, SO2, SOF2, and SO2F2 adsorption on the In2O3-ZnO heterojunction surface.In the H2S adsorption system shown in Figure 3(a1,a2), there is no significant change in TDOS.It can also be observed from PDOS that the hybridization phenomenon between atomic orbitals is very weak.This phenomenon indicates that the conductivity of the system remains almost unchanged before and after H2S adsorption.Figure 3(b1,b2) shows the TDOS and PDOS of the SO2 gas adsorption system.It can be observed that TDOS almost completely overlaps above the Fermi level.However, the overall density of states shows an increasing trend below the Fermi level, with the peak value increasing most significantly around −5 eV.Due to the increase in electron filling probability at this location, the conductivity of the system increases.In the PDOS distribution, there is a very obvious hybridization phenomenon between the 5p orbitals of the In atoms and the 2p orbitals of the S atoms between −10 eV and 5 eV.This phenomenon confirms the strong chemical interaction between the SO2 and In2O3-ZnO heterojunction, as well as the increase in TDOS near −5 eV.During the adsorption process of SOF2 shown in Figure 3(c1,c2), TDOS remains almost unchanged, which is consistent with its small adsorption energy and small charge transfer performance.In Figure 3(d1,d2), TDOS also keeps overlapping before and after the adsorption of SO2F2 gas, except in two new small peaks, which appear near −10 eV.In the PDOS analysis of the SO2F2 adsorption process, only In-5s orbitals and F-2p orbitals have a certain degree of hybridization, but the hybridization is not significant, which has a certain impact on improving gas adsorption performance.This result explains that compared to SOF2 gas molecules, the SO2F2 gas adsorption system has a greater adsorption energy.
To further understand the adsorption mechanism of the In2O3-ZnO heterojunction on the four SF6 decomposition gasses, the charge difference density before and after adsorption were studied as shown in Figure 4; the increase in charge density is shown in red, while the decrease is shown in blue.For the SOF2 and SO2F2 gas molecules adsorption, there is almost no charge transfer between the In2O3-ZnO heterojunction and the gas molecules.When adsorbing H2S gas molecules, the S atom acts as an electron acceptor, indicating a decrease in the number of charges around the S atom.However, the red color around the S atom is very weak, and the entire H2S gas molecule transfers 0.255 |e| to the In2O3-ZnO heterojunction substrate during the adsorption process.When adsorbing SO2 gas molecules, the charge density around the O atom of SO2 molecules significantly increases, indicating that the charge transfers from the In2O3-ZnO heterojunction to SO2.

• DOS, CDD, and molecular orbital analysis
Figure 3 shows the density of states distribution of H 2 S, SO 2 , SOF 2 , and SO 2 F 2 adsorption on the In 2 O 3 -ZnO heterojunction surface.In the H 2 S adsorption system shown in Figure 3(a1,a2), there is no significant change in TDOS.It can also be observed from PDOS that the hybridization phenomenon between atomic orbitals is very weak.This phenomenon indicates that the conductivity of the system remains almost unchanged before and after H 2 S adsorption.Figure 3(b1,b2) shows the TDOS and PDOS of the SO 2 gas adsorption system.It can be observed that TDOS almost completely overlaps above the Fermi level.However, the overall density of states shows an increasing trend below the Fermi level, with the peak value increasing most significantly around −5 eV.Due to the increase in electron filling probability at this location, the conductivity of the system increases.In the PDOS distribution, there is a very obvious hybridization phenomenon between the 5p orbitals of the In atoms and the 2p orbitals of the S atoms between −10 eV and 5 eV.This phenomenon confirms the strong chemical interaction between the SO 2 and In 2 O 3 -ZnO heterojunction, as well as the increase in TDOS near −5 eV.During the adsorption process of SOF 2 shown in Figure 3(c1,c2), TDOS remains almost unchanged, which is consistent with its small adsorption energy and small charge transfer performance.In Figure 3(d1,d2), TDOS also keeps overlapping before and after the adsorption of SO 2 F 2 gas, except in two new small peaks, which appear near −10 eV.In the PDOS analysis of the SO 2 F 2 adsorption process, only In-5s orbitals and F-2p orbitals have a certain degree of hybridization, but the hybridization is not significant, which has a certain impact on improving gas adsorption performance.This result explains that compared to SOF 2 gas molecules, the SO 2 F 2 gas adsorption system has a greater adsorption energy.
To further understand the adsorption mechanism of the In 2 O 3 -ZnO heterojunction on the four SF 6 decomposition gasses, the charge difference density before and after adsorption were studied as shown in Figure 4; the increase in charge density is shown in red, while the decrease is shown in blue.For the SOF 2 and SO 2 F 2 gas molecules adsorption, there is almost no charge transfer between the In 2 O 3 -ZnO heterojunction and the gas molecules.When adsorbing H 2 S gas molecules, the S atom acts as an electron acceptor, indicating a decrease in the number of charges around the S atom.However, the red color around the S atom is very weak, and the entire H 2 S gas molecule transfers 0.255 |e| to the In 2 O 3 -ZnO heterojunction substrate during the adsorption process.When adsorbing SO 2 gas molecules, the charge density around the O atom of SO 2 molecules significantly increases, indicating that the charge transfers from the In 2 O 3 -ZnO heterojunction to SO 2 .This is attributed to the formation of chemical bonds between gas molecules and the substrate, as well as the large adsorption energy.The In 2 O 3 -ZnO heterojunction has a good adsorption effect on the SO 2 gas molecule, but this adsorption force is not sufficient to break the original chemical bonds of the SO 2 gas molecules.Therefore, the SO 2 gas molecules have the potential for desorption.This is attributed to the formation of chemical bonds between gas molecules and the substrate, as well as the large adsorption energy.The In2O3-ZnO heterojunction has a good adsorption effect on the SO2 gas molecule, but this adsorption force is not sufficient to break the original chemical bonds of the SO2 gas molecules.Therefore, the SO2 gas molecules have the potential for desorption.

In-5s
In-5p S-2p O-2p In-5p In-5s In-5s In-5p  Figure 5 shows the changes in the frontier molecular orbitals of theIn2O3-ZnO heterojunction before and after H2S, SO2, SOF2, and SO2F2 molecules adsorption, where LUMO and HOMO represent the lowest unoccupied molecular orbitals and the highest occupied molecular orbitals, respectively.It can be observed that after gas molecule adsorption, the molecular orbitals and corresponding energies change.Usually, changes in the energy gap can directly affect conductivity.During the adsorption process of H2S and SO2, there is a significant change in the energy gap value, which is related to chemical adsorption.Specifically, the conductivity of the H2S adsorption system increases, while This is attributed to the formation of chemical bonds between gas molecules and the sub strate, as well as the large adsorption energy.The In2O3-ZnO heterojunction has a good adsorption effect on the SO2 gas molecule, but this adsorption force is not sufficient to break the original chemical bonds of the SO2 gas molecules.Therefore, the SO2 gas mole cules have the potential for desorption.

In-5s
In-5p S-2p O-2p In-5p In-5s In-5s In-5p Figure 5 shows the changes in the frontier molecular orbitals of theIn2O3-ZnO het erojunction before and after H2S, SO2, SOF2, and SO2F2 molecules adsorption, wher LUMO and HOMO represent the lowest unoccupied molecular orbitals and the highes occupied molecular orbitals, respectively.It can be observed that after gas molecule ad sorption, the molecular orbitals and corresponding energies change.Usually, changes in the energy gap can directly affect conductivity.During the adsorption process of H2S and SO2, there is a significant change in the energy gap value, which is related to chemica adsorption.Specifically, the conductivity of the H2S adsorption system increases, whil Figure 5 shows the changes in the frontier molecular orbitals of theIn 2 O 3 -ZnO heterojunction before and after H 2 S, SO 2 , SOF 2 , and SO 2 F 2 molecules adsorption, where LUMO and HOMO represent the lowest unoccupied molecular orbitals and the highest occupied molecular orbitals, respectively.It can be observed that after gas molecule adsorption, the molecular orbitals and corresponding energies change.Usually, changes in the energy gap can directly affect conductivity.During the adsorption process of H 2 S and SO 2 , there is a significant change in the energy gap value, which is related to chemical adsorption.Specifically, the conductivity of the H 2 S adsorption system increases, while the conductivity of the SO 2 adsorption system decreases.After the adsorption of SOF 2 and SO 2 F 2 gas molecules, the energy gap increases, indicating a decrease in conductivity.Although this change is not very obvious, it is consistent with the previous analysis results.According to the extent of the energy gap change, the conductivity changes caused by gas adsorption are ranked in descending order: SO 2 > H 2 S > SO 2 F 2 > SOF 2 .
the conductivity of the SO2 adsorption system decreases.After the adsorption of SOF2 and SO2F2 gas molecules, the energy gap increases, indicating a decrease in conductivity.Alt hough this change is not very obvious, it is consistent with the previous analysis results According to the extent of the energy gap change, the conductivity changes caused by gas adsorption are ranked in descending order: SO2 > H2S > SO2F2 > SOF2.This section calculated and analyzed the adsorption performance of the In2O3-ZnO heterojunction on four gas molecules: H2S, SO2, SOF2, and SO2F2.Firstly, the most stable heterostructure and gas adsorption model were structured; each structure has a negative binding or adsorption energy, indicating a spontaneously performed reaction.There is a formation of chemical bonds in the adsorption process of H2S and SO2, indicating a chem ical adsorption, while the adsorption of SOF2 and SO2F2 belongs to physical adsorption with small adsorption energy and longer adsorption distance.According to the density o states and molecular orbital theory analysis, the conductivity of the H2S adsorption system increases, while the conductivity of the SO2 adsorption system decreases.However, in the H2S adsorption system, the structure of gas molecules is disrupted, and cannot be de sorbed from the In2O3-ZnO heterojunction surface.In the SO2 adsorption system, the orig inal chemical bonds of SO2 gas molecules keep intact, showing a certain desorption po tential.Therefore, the In2O3-ZnO heterojunction can serve as an adsorbent material for H2S gas molecules, while it can also be used as a gas-sensitive material for SO2 gas molecules  This section calculated and analyzed the adsorption performance of the In 2 O 3 -ZnO heterojunction on four gas molecules: H 2 S, SO 2 , SOF 2 , and SO 2 F 2 .Firstly, the most stable heterostructure and gas adsorption model were structured; each structure has a negative binding or adsorption energy, indicating a spontaneously performed reaction.There is a formation of chemical bonds in the adsorption process of H 2 S and SO 2 , indicating a chemical adsorption, while the adsorption of SOF 2 and SO 2 F 2 belongs to physical adsorption with small adsorption energy and longer adsorption distance.According to the density of states and molecular orbital theory analysis, the conductivity of the H 2 S adsorption system increases, while the conductivity of the SO 2 adsorption system decreases.However, in the H 2 S adsorption system, the structure of gas molecules is disrupted, and cannot be desorbed from the In 2 O 3 -ZnO heterojunction surface.In the SO 2 adsorption system, the original chemical bonds of SO 2 gas molecules keep intact, showing a certain desorption potential.Therefore, the In 2 O 3 -ZnO heterojunction can serve as an adsorbent material for H 2 S gas molecules, while it can also be used as a gas-sensitive material for SO 2 gas molecules.

Adsorption Properties of TiO 2 -ZnO Heterojunction on SF 6 Decomposition Gases 2.2.1. Construction and Energy Band Analysis of TiO 2 -ZnO Heterojunction Model
This section constructs and optimizes the most stable model by combining the TiO 2 and ZnO heterojunction crystal planes.Firstly, the TiO 2 crystal plane and ZnO heterojunction crystal plane were constructed and optimized separately.The lattice parameters of the TiO 2 crystal were as follows: a = b = 3.776 Å, c = 9.486 Å, γ = 90º.The ZnO heterojunction crystal's lattice parameters were as follows: a = b = 3.249 Å, c = 5.205 Å, γ = 90º.Then, the TiO 2 and ZnO heterojunction crystal planes were cut and expanded, and the obtained parameters were as follows: a = 18.880Å, b = 10.210Å, γ = 90º (TiO 2 heterojunction); a = 19.496Å, b = 10.411Å, γ = 90º (ZnO heterojunction).In this model, the lower layer is the TiO 2 layer, and the upper layer is the ZnO heterojunction layer.There are a total of 156 atoms, including 96 atoms in the ZnO layer and 60 atoms in the TiO 2 layer.Figure 6a,b shows the structure of TiO 2 -ZnO heterojunction after structural optimization, with a binding energy of 12.819 eV.In the model, the TiO 2 layer and the ZnO layer are connected by chemical bonds.In addition, the formation of chemical bonds makes it easier for electrons to migrate from the ZnO heterojunction layer to the TiO 2 layer, as evidenced by the small energy band (1.292 eV), shown in Figure 6c.Therefore, the conductivity of the TiO 2 -ZnO heterojunction is enhanced.shows the structure of TiO2-ZnO heterojunction after structural optimization, with a binding energy of 12.819 eV.In the model, the TiO2 layer and the ZnO layer are connected by chemical bonds.In addition, the formation of chemical bonds makes it easier for electrons to migrate from the ZnO heterojunction layer to the TiO2 layer, as evidenced by the small energy band (1.292 eV), shown in Figure 6c.Therefore, the conductivity of the TiO2-ZnO heterojunction is enhanced.

Adsorption Properties of TiO2-ZnO Heterojunction 
Gas adsorption structure analysis As shown in Figure 7, the most stable adsorption models are calculated by adjusting the various adsorption sites of gas molecules.As listed in Table 2, the H2S gas adsorption system has the maximum adsorption (−0.788 eV) and the maximum charge transfer amount (0.249 |e|).From Figure 7(a1,a2), it can be observed that the shortest distance between the S atoms and Ti atoms is 2.780 Å, and the long adsorption distance makes it impossible to form a chemical bond between the two.In the adsorption system of the TiO2-ZnO heterojunction for the four gasses, only chemical bonds form between the SO2 gas molecule and the substrate material, while the other three gas molecules still maintained a relatively long adsorption distance from the TiO2-ZnO heterojunction substrate.In the adsorption system of SOF2 and SO2F2, they have a long adsorption distance, small adsorption energy, and a small amount of charge transfer.Therefore, the adsorption of SOF2 and SO2F2 gas molecules on the TiO2-ZnO heterojunction is physical adsorption.In addition, only the charge transfer amount of the SO2 adsorption system is negative, signifying that gas molecules act as electron acceptors to obtain electrons from the substrate.It is the formation of chemical bonds (S-O: 1.766 Å; Ti-O: 1.958 Å) that facilitate electron migration, allowing SO2 gas molecules to form a whole with the TiO2 layer, and obtain more electrons from the ZnO heterojunction layer.In the other three adsorption systems, the TiO2 layer acts as a stronger electron acceptor, causing gas molecules to lose electrons.In the above gas adsorption, the molecular structure of the gas molecules is not disrupted.Therefore, under certain conditions, the four gas molecules can desorb from the surface of the TiO2-ZnO heterojunction.

Adsorption Properties of TiO 2 -ZnO Heterojunction
• Gas adsorption structure analysis As shown in Figure 7, the most stable adsorption models are calculated by adjusting the various adsorption sites of gas molecules.As listed in Table 2, the H 2 S gas adsorption system has the maximum adsorption (−0.788 eV) and the maximum charge transfer amount (0.249 |e|).From Figure 7(a1,a2), it can be observed that the shortest distance between the S atoms and Ti atoms is 2.780 Å, and the long adsorption distance makes it impossible to form a chemical bond between the two.In the adsorption system of the TiO 2 -ZnO heterojunction for the four gasses, only chemical bonds form between the SO 2 gas molecule and the substrate material, while the other three gas molecules still maintained a relatively long adsorption distance from the TiO 2 -ZnO heterojunction substrate.In the adsorption system of SOF 2 and SO 2 F 2 , they have a long adsorption distance, small adsorption energy, and a small amount of charge transfer.Therefore, the adsorption of SOF 2 and SO 2 F 2 gas molecules on the TiO 2 -ZnO heterojunction is physical adsorption.In addition, only the charge transfer amount of the SO 2 adsorption system is negative, signifying that gas molecules act as electron acceptors to obtain electrons from the substrate.It is the formation of chemical bonds (S-O: 1.766 Å; Ti-O: 1.958 Å) that facilitate electron migration, allowing SO 2 gas molecules to form a whole with the TiO 2 layer, and obtain more electrons from the ZnO heterojunction layer.In the other three adsorption systems, the TiO 2 layer acts as a stronger electron acceptor, causing gas molecules to lose electrons.In the above gas adsorption, the molecular structure of the gas molecules is not disrupted.Therefore, under certain conditions, the four gas molecules can desorb from the surface of the TiO 2 -ZnO heterojunction.

 DOS, CDD, and molecular orbital analysis
To investigate the gas-sensing performance of the TiO2-ZnO heterojunction on H2S, SO2, SOF2, and SO2F2 gas molecules, the density of states distribution of the four gas adsorption systems was analyzed, as shown in Figure 8.There is no significant change in TDOS before and after the four gas molecules' adsorption on the surface of the TiO2-ZnO heterojunction.From PDOS analysis, it can be seen that the degree of atomic orbital hybridization is very weak.Therefore, it is not possible to accurately determine the changes in conductivity from the density of states analysis.Figure 9 shows the distribution of charges upon gas adsorption.From Figure 9a, it can be observed that the electron density around H2S gas is relatively low, which is consistent with the conclusion that the H2S molecule acts as an electron donor.In Figure 9b, the blue color is around the S atom of the SO2 gas molecule and the red color is around the O atoms of the TiO2 layer, indicating that SO2 receives electrons from the TiO2 layer.This conclusion stays consistent with the charge transfer of SO2 of−0.142|e| during the adsorption process.In the SOF2 and SO2F2 adsorption systems, there is no significant change in the distribution of electrons, indicating a small amount of charge transfer.

• DOS, CDD, and molecular orbital analysis
To investigate the gas-sensing performance of the TiO 2 -ZnO heterojunction on H 2 S, SO 2 , SOF 2 , and SO 2 F 2 gas molecules, the density of states distribution of the four gas adsorption systems was analyzed, as shown in Figure 8.There is no significant change in TDOS before and after the four gas molecules' adsorption on the surface of the TiO 2 -ZnO heterojunction.From PDOS analysis, it can be seen that the degree of atomic orbital hybridization is very weak.Therefore, it is not possible to accurately determine the changes in conductivity from the density of states analysis.Figure 9 shows the distribution of charges upon gas adsorption.From Figure 9a, it can be observed that the electron density around H 2 S gas is relatively low, which is consistent with the conclusion that the H 2 S molecule acts as an electron donor.In Figure 9b, the blue color is around the S atom of the SO 2 gas molecule and the red color is around the O atoms of the TiO 2 layer, indicating that SO 2 receives electrons from the TiO 2 layer.This conclusion stays consistent with the charge transfer of SO 2 of−0.142|e| during the adsorption process.In the SOF 2 and SO 2 F 2 adsorption systems, there is no significant change in the distribution of electrons, indicating a small amount of charge transfer.

 DOS, CDD, and molecular orbital analysis
To investigate the gas-sensing performance of the TiO2-ZnO heterojunction on H2S, SO2, SOF2, and SO2F2 gas molecules, the density of states distribution of the four gas adsorption systems was analyzed, as shown in Figure 8.There is no significant change in TDOS before and after the four gas molecules' adsorption on the surface of the TiO2-ZnO heterojunction.From PDOS analysis, it can be seen that the degree of atomic orbital hybridization is very weak.Therefore, it is not possible to accurately determine the changes in conductivity from the density of states analysis.Figure 9 shows the distribution of charges upon gas adsorption.From Figure 9a, it can be observed that the electron density around H2S gas is relatively low, which is consistent with the conclusion that the H2S molecule acts as an electron donor.In Figure 9b, the blue color is around the S atom of the SO2 gas molecule and the red color is around the O atoms of the TiO2 layer, indicating that SO2 receives electrons from the TiO2 layer.This conclusion stays consistent with the charge transfer of SO2 of−0.142|e| during the adsorption process.In the SOF2 and SO2F2 adsorption systems, there is no significant change in the distribution of electrons, indicating a small amount of charge transfer.Figure 10 shows the frontier molecular orbitals' distribution and the related energ of the gas molecules before and after adsorption on the surface of the TiO2-ZnO hetero junction.Before gas adsorption, LUMO is mainly distributed on the TiO2 layer, whil HOMO is mainly distributed on the ZnO heterojunction layer, and both are evenly dis tributed.The energy corresponding to LUMO is −4.921 eV, and the energy correspondin to HOMO is −6.213 eV.In the three gas adsorption systems of H2S, SOF2, and SO2F2, th energy gap only undergoes slight changes, within a range of less than 0.03 eV.The con ductivity of the H2S and SOF2 gas adsorption systems increases, while the conductivity o the SO2F2 gas adsorption systems decreases.In the SO2 gas adsorption system, the distri bution of LUMO on the TiO2 layer becomes thinner, followed by a significant increase i the energy gap value, which is 0.086 eV.This means there is a significant increase in con ductivity after gas adsorption, providing a theoretical basis for the development of resis tive chemical sensors.This section investigated the gas-sensing response of TiO2-ZnO heterojunctions t four gas molecules.Based on DFT calculations, the most stable heterostructure and ga adsorption model were calculated, and the gas-sensing performance of the TiO2-ZnO het erojunction heterostructure on four gas molecules was explored by analyzing density o states, CDD, and the HOMO-LUMO gap.The calculation results show that the TiO2 an Figure 10 shows the frontier molecular orbitals' distribution and the related energy of the gas molecules before and after adsorption on the surface of the TiO 2 -ZnO heterojunction.Before gas adsorption, LUMO is mainly distributed on the TiO 2 layer, while HOMO is mainly distributed on the ZnO heterojunction layer, and both are evenly distributed.The energy corresponding to LUMO is −4.921 eV, and the energy corresponding to HOMO is −6.213 eV.In the three gas adsorption systems of H 2 S, SOF 2 , and SO 2 F 2 , the energy gap only undergoes slight changes, within a range of less than 0.03 eV.The conductivity of the H 2 S and SOF 2 gas adsorption systems increases, while the conductivity of the SO 2 F 2 gas adsorption systems decreases.In the SO 2 gas adsorption system, the distribution of LUMO on the TiO 2 layer becomes thinner, followed by a significant increase in the energy gap value, which is 0.086 eV.This means there is a significant increase in conductivity after gas adsorption, providing a theoretical basis for the development of resistive chemical sensors.Figure 10 shows the frontier molecular orbitals' distribution and the related energy of the gas molecules before and after adsorption on the surface of the TiO2-ZnO hetero junction.Before gas adsorption, LUMO is mainly distributed on the TiO2 layer, while HOMO is mainly distributed on the ZnO heterojunction layer, and both are evenly dis tributed.The energy corresponding to LUMO is −4.921 eV, and the energy corresponding to HOMO is −6.213 eV.In the three gas adsorption systems of H2S, SOF2, and SO2F2, the energy gap only undergoes slight changes, within a range of less than 0.03 eV.The con ductivity of the H2S and SOF2 gas adsorption systems increases, while the conductivity o the SO2F2 gas adsorption systems decreases.In the SO2 gas adsorption system, the distri bution of LUMO on the TiO2 layer becomes thinner, followed by a significant increase in the energy gap value, which is 0.086 eV.This means there is a significant increase in con ductivity after gas adsorption, providing a theoretical basis for the development of resis tive chemical sensors.This section investigated the gas-sensing response of TiO2-ZnO heterojunctions to four gas molecules.Based on DFT calculations, the most stable heterostructure and gas adsorption model were calculated, and the gas-sensing performance of the TiO2-ZnO het erojunction heterostructure on four gas molecules was explored by analyzing density o states, CDD, and the HOMO-LUMO gap.The calculation results show that the TiO2 and This section investigated the gas-sensing response of TiO 2 -ZnO heterojunctions to four gas molecules.Based on DFT calculations, the most stable heterostructure and gas adsorption model were calculated, and the gas-sensing performance of the TiO 2 -ZnO heterojunction heterostructure on four gas molecules was explored by analyzing density of states, CDD, and the HOMO-LUMO gap.The calculation results show that the TiO 2 and ZnO crystal planes can form stable heterojunctions with high formation energy.The adsorption type of H 2 S, SOF 2 , and SO 2 F 2 by the TiO 2 -ZnO heterojunction is physical adsorption, while the adsorption type of SO 2 is chemical adsorption.The conductivity of the TiO 2 -ZnO heterojunction increases after adsorbing H 2 S and SOF 2 gas molecules; after adsorbing SO 2 and SO 2 F 2 , there is a decrease in conductivity.Specifically, the change in conductivity is most significant after the adsorption of SO 2 , indicating that the TiO 2 -ZnO heterojunction can effectively detect SO 2 gas.

Methods
All calculations were performed based on DFT [27].Specifically, the generalized gradient approximation method with a GGA-PBE function was used for geometric optimization and energy calculation [28], while the Tkatchenko-Scheffler method was used to correct the van der Waals force to obtain accurate results [29].In addition, a DFT semi-core pseudopotential (DSSP) was adopted and its value was set to 6 [30].The K-points in the Brillouin area were set to 5 × 5 × 1.The energy convergence accuracy, maximum stress, and displacement were set to 2.721 × 10 −4 eV, 0.109 eV/Å, and 0.005 Å, respectively [31,32].To eliminate the influence of interlayer interactions, the thickness of the vacuum layer was set to 20 Å.

Conclusions
In order to achieve the online monitoring of GIS faults, this study analyzed the adsorption and gas-sensing properties of In 2 O 3 -ZnO and TiO 2 -ZnO heterojunctions on four SF 6 decomposition gasses (H 2 S, SO 2 , SOF 2 , and SO 2 F 2 ).Compared to intrinsic In 2 O 3 and ZnO, the band value of the In 2 O 3 -ZnO heterojunction significantly decreases.The energy band value of the TiO 2 -ZnO heterojunction is larger than that of the In 2 O 3 -ZnO heterojunction.The In 2 O 3 -ZnO heterojunction shows large adsorption energy for H 2 S and SO 2 via chemical adsorption, and small adsorption energy for SOF 2 and SO 2 F 2 via physical adsorption.After gas adsorption, the conductivity of the H 2 S adsorption system increases, while the conductivity of the SO 2 adsorption system decreases.The In 2 O 3 -ZnO heterojunction can serve as an adsorbent material for H 2 S gas molecules, while it can also be used as a gas-sensitive material for SO 2 gas molecules.While TiO 2 -ZnO only shows chemical adsorption for SO 2 , there is a distinct change in the conductivity of the adsorption system.The research results play an important role in exploring novel gassensitive materials and preparing a specific gas sensor using the online monitoring of power insulation equipment.

2 . 1 .
Adsorption Properties of In 2 O 3 -ZnO Heterojunction on SF 6 Decomposition Gases 2.1.1.Construction and Energy Band Analysis of In 2 O 3 -ZnO Heterojunction Model In the section, the most stable structure of the In 2 O 3 -ZnO heterojunction model was constructed and optimized, as shown in Figure 1a,b.The In 2 O 3 -ZnO heterojunction contains a total of 120 atoms, including 40 atoms in the In 2 O 3 layer and 80 atoms in the ZnO heterojunction layer.The ZnO heterojunction crystal plane is placed in the lower layer, and the In 2 O 3 crystal plane is placed in the upper layer.The lattice parameters of the ZnO heterojunction are as follows: a = 11.2558Å, b = 16.2464Å, and γ = 90º.The lattice parameters of In 2 O 3 are as follows: a = 11.0324Å, b = 16.0283Å, and γ = 90º.The lattice parameters of the constructed In 2 O 3 -ZnO heterojunction are as follows: a = 11.1441Å, b = 16.1373Å, and γ = 90º.
Figure2shows the most stable adsorption structures for H2S, SO2, SOF2, and SO2F2 on the In2O3-ZnO heterojunction surface, and Table1lists the corresponding adsorption parameters.For the H2S adsorption system shown in Figure2(a1,a2), the strong binding force between the O and H atoms leads to the breaking of the H-S bond, resulting in the formation of a new H-O bond (1.008 Å); here, a new chemical bond is formed between the S atom and the nearest In atom with a length of 2.486 Å.As listed in Table 1, the adsorption energy of the H2S adsorption system reaches −1.924 eV, indicating chemical adsorption.H2S molecules act as electron contributors and transfer the 0.255 |e| charge to the In2O3-ZnO heterojunction, which is consistent with the reducibility properties of H2S.As shown in Figure 2 (b1,b2), the O atoms of the SO2 molecule form chemical bonds with In atoms, while the S atom of the SO2 molecule forms chemical bonds with O atoms, resulting in varying degrees of elongation of the S-O bond in SO2.Meanwhile, the system has an adsorption energy of −1.992 eV and a charge transfer capacity of −0.361 |e|, indicating a strong binding force between the SO2 molecules and the In2O3-ZnO heterojunction.For the adsorption systems of SOF2 and SO2F2, shown in Figure 2(c1,c2) and Figure2(d1,d2), there is a significant distance between the gas molecules and heterojunctions; gas molecules can easily desorb from the surface of the In2O3-ZnO heterojunction, indicating that the adsorption process is physical adsorption.In addition, the small adsorption energy (SOF2: Eads = −0.465eV, SO2F2: Eads = −0.501eV) and the small amount of charge transfer (SOF2: Qt = 0.016 |e|, SO2F2: Qt = 0.011 |e|) also confirm this physical adsorption effect.In summary, the adsorption process of H2S and SO2 is chemical adsorption, and the molecular structure of the H2S gas molecules is destroyed, making it difficult to carry out the desorption process.The adsorption process of SOF2 and SO2F2 is physical adsorption and the adsorption energy is moderate, which together can have a certain adsorption capacity for gas molecules and make desorption possible.
Figure2shows the most stable adsorption structures for H 2 S, SO 2 , SOF 2 , and SO 2 F 2 on the In 2 O 3 -ZnO heterojunction surface, and Table1lists the corresponding adsorption parameters.For the H 2 S adsorption system shown in Figure2(a1,a2), the strong binding force between the O and H atoms leads to the breaking of the H-S bond, resulting in the formation of a new H-O bond (1.008 Å); here, a new chemical bond is formed between the S atom and the nearest In atom with a length of 2.486 Å.As listed in Table 1, the adsorption energy of the H 2 S adsorption system reaches −1.924 eV, indicating chemical adsorption.H 2 S molecules act as electron contributors and transfer the 0.255 |e| charge to the In 2 O 3 -ZnO heterojunction, which is consistent with the reducibility properties of H 2 S. As shown in Figure 2(b1,b2), the O atoms of the SO 2 molecule form chemical bonds with In atoms, while the S atom of the SO 2 molecule forms chemical bonds with O atoms, resulting in varying degrees of elongation of the S-O bond in SO 2 .Meanwhile, the system has an adsorption energy of −1.992 eV and a charge transfer capacity of −0.361 |e|, indicating a strong binding force between the SO 2 molecules and the In 2 O 3 -ZnO heterojunction.For the adsorption systems of SOF 2 and SO 2 F 2 , shown in Figure2(c1,c2,d1,d2), there is a significant distance between the gas molecules and heterojunctions; gas molecules can easily desorb from the surface of the In 2 O 3 -ZnO heterojunction, indicating that the adsorption process is physical adsorption.In addition, the small adsorption energy (SOF 2 : E ads = −0.465eV, SO 2 F 2 : E ads = −0.501eV) and the small amount of charge transfer (SOF 2 : Q t = 0.016 |e|, SO 2 F 2 : Q t = 0.011 |e|) also confirm this physical adsorption effect.In summary, the adsorption process of H 2 S and SO 2 is chemical adsorption, and the molecular structure of the H 2 S gas molecules is destroyed, making it difficult to carry out the desorption process.The adsorption process of SOF 2 and SO 2 F 2 is physical adsorption and the adsorption energy is moderate, which together can have a certain adsorption capacity for gas molecules and make desorption possible.

2. 2 . 1 .
Construction and Energy Band Analysis of TiO2-ZnO Heterojunction Model This section constructs and optimizes the most stable model by combining the TiO and ZnO heterojunction crystal planes.Firstly, the TiO2 crystal plane and ZnO heterojunc tion crystal plane were constructed and optimized separately.The lattice parameters o the TiO2 crystal were as follows: a = b = 3.776 Å, c = 9.486 Å, γ= 90 º.The ZnO heterojunction crystal's lattice parameters were as follows: a = b = 3.249 Å, c = 5.205 Å, γ= 90 º.Then, the TiO2 and ZnO heterojunction crystal planes were cut and expanded, and the obtained pa rameters were as follows: a = 18.880Å, b = 10.210Å, γ= 90 º (TiO2 heterojunction); a = 19.496Å, b = 10.411Å, γ= 90 º (ZnO heterojunction).In this model, the lower layer is the TiO2 layer, and the upper layer is the ZnO heterojunction layer.There are a total of 156

Figure 5 .
Figure 5. Calculated frontier molecular orbitals, HOMO and LUMO, before and after gas adsorption on In 2 O 3 -ZnO heterojunction.

Figure 10 .
Figure 10.Calculated frontier molecular orbitals, HOMO and LUMO, before and after gas adsorption on TiO 2 -ZnO heterojunction.

Table 1 .
The 2.1.2.Adsorption Properties of In 2 O 3 -ZnO Heterojunction•Gas adsorption structure analysisFigure2shows the most stable adsorption structures for H 2 S, SO 2 , SOF 2 , and SO 2 F 2 on the In 2 O 3 -ZnO heterojunction surface, and Table1lists the corresponding adsorption parameters.For the H 2 S adsorption system shown in Figure2(a1,a2), the strong binding force between the O and H atoms leads to the breaking of the H-S bond, resulting in the formation of a new H-O bond (1.008 Å); here, a new chemical bond is formed between the S atom and the nearest In atom with a length of 2.486 Å.As listed in Table1, the adsorption energy of the H 2 S adsorption system reaches −1.924 eV, indicating chemical adsorption.

Table 1 .
The adsorption parameters of H 2 S, SO 2 , SOF 2 , and SO 2 F 2 on In 2 O 3 -ZnO heterojunction surface.